The present invention relates to beam splitting systems for splitting incident light, and measuring systems using the beam splitting systems. The inventive beam splitting system is suitable for a light intensity control unit for monitoring a light intensity of an ultraviolet (UV) pulse laser, such as an excimer laser as a light source.
The present invention also relates to measuring systems utilizing optical mechanisms, and more particularly to an apparatus for measuring the transmittance of a sample using the UV light. The present invention is suitable, for example, for an apparatus for measuring the transmittance of an optical element using a UV pulse laser as a light source.
Along with the recent demand on smaller and lower profile electronic devices, minute semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. In order to meet this demand, various proposals have been made to improve exposure resolution. The quality of an image to be transferred onto a wafer, etc. is significantly affected by the illumination performance, e.g., distributions of illumination on the mask and wafer planes. Therefore, in order to provide high quality semiconductor wafers, LCDs, and thin-film magnetic heads, accurate exposure amount control is necessary.
An exposure amount control apparatus typically splits light from a light source using a half mirror, etc., receives it via a light receiving element, and feedback-controls a light intensity of the light source so that a light intensity fluctuation at the illuminated area may fall within a permissible range. The half mirror is provided, for example, after an optical integrator, in a position equivalent to a pattern on a reticle or mask (these terms are interchangeably used in this application).
Use of a shorter wavelength has been promoted to improve resolution. The light source changes from a KrF excimer laser (with a wavelength of about 248 nm) to an ArF excimer laser (with a wavelength of about 193 nm). An F2 excimer laser (with a wavelength of about 157 nm) is about to be put to practical use. An optical element in an optical system in the exposure apparatus requires the high transmission property for UV light supplied from the light source and high resistance to the UV light so that it seldom attenuates transmittance for long-time exposure. The optical element must be fully examined for its transmission property and UV resistance, and thus its transmittance has been measured frequently.
A transmittance measurement apparatus typically splits a UV laser beam of an object excimer laser using a half mirror by reflecting and transmitting it, receives a reflected beam (a reference beam) by one sensor, receives the transmitted beam (or tested beam) by another sensor after the transmitted beam transmits through a sample, and measures the sample's transmittance by calculating a ratio between light intensities detected with and without the sample.
On the other hand, an exposure amount control apparatus typically splits light from the light source using a half mirror, receives it via a sensor, and feeds back the light intensity of the light source so that fluctuations in the light intensity in the illumination area may fall within a permissible range. A half mirror is provided, for example, after an optical integrator, in a position equivalent to a pattern on a reticle.
A conventional exposure amount control apparatus and transmittance measurement apparatus disadvantageously cannot accurately detect the light intensity, resulting in insufficient exposure amount control and transmittance measurement. As well, they cannot provide high quality devices with good exposure performance such as a throughput.
As a result of eager studies over causes of this problem, the instant inventors have discovered that the conventional erroneous light intensity detection results in polarized fluctuation of laser beam. A half mirror has different reflectance and transmittance for p and s polarization components of the laser beam. The polarizations of two beams split by the half mirror fluctuate when the laser beam (i.e., light incident on the half mirror) has polarized fluctuation. The polarized fluctuation occurs when a polarization component of the laser beam incident upon a half mirror fluctuates in accordance with the oscillation voltage or changes for each pulse. In case, the sensor cannot accurately detect the light intensity of the laser beam because a ratio of light intensities between incident and split light fluctuates due to a fluctuated polarizations between the beam incident on the half mirror and the split light that has passing through the half mirror.